feat: separate cortex-gui into standalone repository (#509)

BREAKING: The cortex-gui (Tauri desktop application) has been moved to a separate repository:
https://github.com/CortexLM/cortex-ide

This allows the GUI to be developed independently and removes the Tauri
dependencies from the CLI workspace. The GUI crate previously depended on:
- cortex-engine
- cortex-protocol
- cortex-storage

These dependencies have been stubbed out in the new repository with TODOs
for future re-integration.

Cargo.toml: Removed cortex-gui/src-tauri from workspace members

Author: echobt

Cargo.lock

@@ -106,7 +106,8 @@ members = [
     # ==========================================================================
     # Desktop Application (GUI) - Tauri
     # ==========================================================================
-    "cortex-gui/src-tauri",
+    # NOTE: cortex-gui has been moved to a separate repository: CortexLM/cortex-ide
+    # See: https://github.com/CortexLM/cortex-ide
 ]
 
 [workspace.package]